The present invention relates to a concrete block molding method and an apparatus for producing multilayer type concrete blocks.
In concrete blocks or concrete products similar to concrete blocks, it is sometimes desired that the surface be smooth, strong, water-repellent and waterproof. In order to comply with such a desire, the density of the surface layer of the concrete product has to be increased and accordingly the water-cement ratio in the surface layer has to be increased. However, increasing the water-cement ratio is not suitable for a speedy ejection of concrete blocks from a molding box. Furthermore, in order to comply with the above-mentioned desire, the mixing ratio of cement in the concrete product has to be increased. This makes the production cost of the concrete blocks high. In order to overcome such problems, a method of producing concrete blocks comprising a surface layer and a back layer whose components are respectively different is proposed.
To be more specific, in the conventional concrete block products made by the above-mentioned method, the mixing ratio of cement and the water-cement ratio in the surface layer of the concrete block products are high, but in the back layer, a sufficient amount of aggregates is employed and the water-cement ratio is low. In other words, the slump of the concrete mix for use in the surface layer of the concrete block products is about 20 cm, while the slump of the concrete mix for use in the back layer is nearly zero. When a conventional concrete block product is molded, usually the concrete block is pressed and cured with the surface layer thereof down on a pallet. In this case, an excessive amount of water contained in the back layer penetrates the surface layer and comes out of the surface layer. The water from the back layer which stays on the pallet is so basic that it forms white patterns on the surface of the concrete block product. This is one of the disadvantages of the conventional method of producing concrete blocks.
Furthermore, when supplying two types of concrete mixes with different mixing ratios of the ingredients into a molding box, as the first layer, a soft kneaded concrete mix is first supplied to the molding box by a concrete mix supply means and after the first layer is supplied uniformly, a hard kneaded concrete mix for forming a back layer of the concrete block product is supplied. In this case, the thickness of the surface layer is apt to be made uneven and occasionally the back layer appears through the thin portions of the surface layer. In order to prevent this, the surface layer has to be made thick enough although comparatively expensive materials, such as ornamental pebbles, white cement, and pigments, are employed in the surface layer. This is another disadvantage of the conventional method of producing concrete blocks.